1. Field of the Invention
The present invention relates to a novel semiconductor for photoelectric conversion use which has excellent optical and electrical properties of both a crystalline semiconductor, in particular, a single crystal semiconductor and a non-crystalline semiconductor, in particular, an amorphous semiconductor.
2. Description of the Prior Art
Heretofore, there have been proposed photoelectric conversion semiconductors which are formed of a single crystal semiconductor, a polycrystalline semiconductor and a non-crystalline semiconductor, respectively.
The single crystal, photoelectric conversion semiconductor usually exhibits a high degree of photo-conductivity as compared with the polycrystalline and non-crystalline photoelectric conversion semiconductors each of which is doped with a dangling bond neutralizer. For example, in the case where the polycrystalline, photoelectric conversion semiconductor is formed of polycrystalline silicon, its photoconductivity is 10.sup.-5 to 10.sup.-8 S.cm.sup.-1 under AMl (100 mW/cm.sup.2) illumination of the sunlight (The same shall apply hereinafter.) and its dark-conductivity is only 10.sup.-7 to 10.sup.-12 S.cm.sup.-1. The non-crystalline photoelectric conversion semiconductor which is formed of amorphous silicon has a photoconductivity as low as 10.sup.-6 to 10.sup.-8 S.cm.sup.-1 and a dark-conductivity as low as 10.sup.-8 to 10.sup.-12 S.cm.sup.-1. In contrast thereto, a photoelectric conversion semiconductor formed of single crystal silicon has a photoconductivity of 10.sup.-3 S.cm.sup.-1 or more and a dark-conductivity of 10.sup.-4 to 10.sup.-6 S. cm.sup.-1.
Accordingly, it can be said that the single crystal, photoelectric conversion semiconductor is excellent in terms of photoconductivity and dark-conductivity as compared with the polycrystalline and non-crystalline photoelectric conversion semiconductors.
In general, the single crystal, photoelectric conversion semiconductor has a large diffusion length of minority carriers than the polycrystalline and non-crystalline photoelectric conversion semiconductors each of which is doped with the dangling bond neutralizer. For instance, in the case of the non-crystalline, photoelectric conversion semiconductor being formed of amorphous silicon, its diffusion length of minority carriers is only 300 to 400 A and also in the case of the polycrystalline, photoelectric conversion semiconductor being formed of polycrystalline silicon, its diffusion length of minority carriers is substantially equal to that of the amorphous silicon. In contrast thereto, in the case of the single crystal, photoelectric conversion semiconductor being formed of single crystal silicon, its diffusion length of minority carriers ranges from 10.sup.2 to 10.sup.4 .mu.m.
Accordingly, it can be said that the single crystal photoelectric conversion semiconductor excels the polycrystalline and non-crystalline photoelectric conversion semiconductors in the diffusion length of minority carriers.
Furthermore, the single crystal, photoelectric conversion semiconductor usually has a higher impurity ionization rate than the polycrystalline and non-crystalline photoelectric conversion semiconductors. The impurity ionization rate herein mentioned is a rate in which ions providing the P or N type conductivity are generated in the semiconductor when it has been doped with a P or N type impurity. For example, the non-crystalline, photoelectric conversion semiconductor which is formed of amorphous silicon has an impurity ionization rate of approximately 0.1%. In the case of the polycrystalline, photoelectric conversion semiconductor, even if it is doped with a P or N type impurity, the impurity is precipitated on the grain boundary and, consequently, ions which provide the P or N type conductivity are difficult to generate in the semiconductor. In contrast thereto, the single crystal, photoelectric conversion semiconductor which is formed of single crystal silicon has an impurity ionization rate close to 100%.
Accordingly, it can be said that the single crystal, photoelectric conversion semiconductor is excellent in terms of impurity ionization rate as compared with the polycrystalline and non-crystalline photoelectric conversion semiconductors.
In general, however, the single crystal photoelectric conversion semiconductor has a smaller optical absorption coefficient than the non-crystalline, photoelectric conversion semiconductor as is the case with the polycrystalline photoelectric conversion semiconductor. For instance, the non-crystalline photoelectric conversion semiconductor which is formed of amorphous silicon exhibits optical absorption coefficients of 4.times.10.sup.5 cm.sup.-1, 1.times.10.sup.5 cm.sup.-1 and 2.times.10.sup.4 cm.sup.-1 for lights having wavelengths of 0.4, 0.5 and 0.6 .mu.m, respectively. In contrast thereto, the single crystal, photoelectric conversion semiconductor which is formed of a single crystal silicon has optical absorption coefficients of 1.times.10.sup.5 cm.sup.-1, 1.times.10.sup.4 cm.sup.-1 and 6.times.10.sup.3 cm.sup.-1 for the lights of the 0.4, 0.5 and 0.6 .mu.m wavelengths, as is the case with the polycrystalline, photoelectric conversion semiconductor.
Accordingly, it cannot be said that the single crystal, photoelectric conversion semiconductor excels the non-crystalline, photoelectric conversion semiconductor in optical absorption coefficient.
Moreover, the single crystal, photoelectric conversion semiconductor usually has a smaller energy band gap than the non-crystalline, photoelectric conversion semiconductor doped with the dangling bond neutralizer, as is the case with the polycrystalline, photoelectric conversion semiconductor similarly doped with the neutralizer. For example, in the case where the non-crystalline, photoelectric conversion semiconductor is formed of amorphous silicon, its energy band gap is in the range of 1.7 to 1.9 eV. In contrast thereto, the single crystal, photoelectric conversion semiconductor which is formed of single crystal silicon has an energy band gap of 1.1 eV as in the case where the abovesaid polycrystalline, photoelectric conversion semiconductor is formed of polycrystalline silicon.
Furthermore, in the single crystal, photoelectric conversion semiconductor, transition of electrons is indirect even at low temperatures.
In general, the non-crystalline, photoelectric conversion semiconductor, in particular, an amorphous, photoelectric conversion semiconductor has a larger optical absorption coefficient than the single crystal and polycrystalline photoelectric conversion semiconductors. For instance, in the case where the non-crystalline, photoelectric conversion semiconductor is formed of amorphous silicon, it exhibits optical absorption coefficients of 4.times.10.sup.5 cm.sup.-1, 1.times.10.sup.5 cm.sup.-1 and 2.times.10.sup.4 cm.sup.-1 for the lights of 0.4, 0.5 and 0.6 .mu.m wavelengths, as described previously; namely, the non-crystalline, photoelectric conversion semiconductor is far larger in the optical absorption coefficient than the single crystal, photoelectric conversion semiconductor which is formed of single crystal silicon.
Accordingly, it can be said that the non-crystalline, especially, amorphous photoelectric conversion semiconductor is excellent in optical absorption coefficient, as compared with the single crystal and polycrystalline photoelectric conversion semiconductors.
Besides, in the case where the non-crystalline, in particular, amorphous, photoelectric conversion semiconductor is doped with the dangling bond neutralizer, it has a larger energy band gap than the single crystal, photoelectric conversion semiconductor and the polycrystalline one doped with the dangling bond neutralizer. For example, above non-crystalline, photoelectric conversion semiconductor which is formed of amorphous silicon has an energy band gap ranging from 1.7 to 1.9 eV. This enery band gap is larger than the energy band gaps of the single crystal, photoelectric conversion semiconductor formed of single crystal silicon and the polycrystalline, photoelectric conversion semiconductor formed of polycrystalline silicon and doped with the dangling bond neutralizer.
Further, in the non-crystalline, especially, amorphous photoelectric conversion semiconductor, the transition of electrons is direct at low temperatures.
As will be appreciated from the above, the single crystal, photoelectric conversion semiconductor is excellent in terms of the photoconductivity, diffusion length of minority carriers and impurity ionization rate but poor in the optical absorption coefficient; therefore, this semiconductor is not suitable for use as a light-receiving, photoelectric conversion semiconductor. Further, the single crystal, photoelectric conversion semiconductor cannot be employed as a light-emitting, photoelectric conversion semiconductor because the transistor of electrons is indirect.
The non-crystalline photoelectric conversion semiconductor, especially, the amorphous one, is excellent in the optical absorption coefficient but poor in the photoconductivity, the diffusion length of minority carriers and the impurity ionization rate; accordingly, this semiconductor is not suitable for use a a light-receiving, photoelectric conversion semiconductor. In addition, since the non-crystalline or amorphous, photoelectric conversion semiconductor permits direct transition of electrons, it is considered that this semiconductor can be used a a light-emitting, photoelectric conversion semiconductor, but it cannot be employed because of its low dark-conductivity and impurity ionization rate.
Moreover, the polycrystalline, photoelectric conversion semiconductor is not suitable at all for use as a light-receiving and a light-emitting photoelectric conversion semiconductor.